Just to check: 6.1 gallons in stock tank is 275 mile range +/- at 45 mpg US. (Optomistic, but some have gotten 48 mpg US average on long trips.)

13" x 23" x 4.5" is a volume of .7786 CF and at 7.48 gal/CF encloses a volume of 5.82 gallons, or 0.82 more than the 5.0 reported above. Is some of this space occupied by more than baffles? Or is this the "air gap" mentioned in your text?

The dimensions you cite are the outside measurements and the wall thickness is .125. In addition to the baffles there are two 1 inch tubes that are the rear mounts. Couple those three items with the air space and we have a tank with slightly less than 5 gallons, and well within the IBA/IBR rules.